Heat-sensitive recording materials comprising an electron donating colorless dye (color former) and an electron accepting compound (color developer) have been disclosed in Japanese Patent Publications 14039/70 (corresponding to U.S. Pat. No. 3,539,375) and 4160/68 (corresponding to U.S. patent application Ser. No. 512,546). Such heat-sensitive recording materials must have at least the following characteristics: (1) the color density and the color sensitivity must be sufficiently high, (2) fogging (coloration during preservation before using) is not caused and (3) the colors developed have sufficient fastness. However, present recording materials do not completely satisfy these requirements.
Recently, studies concerning the above described requirement (1) have been carried out due to recently developed high speed heat-sensitive recording systems.
Examples of the electron accepting compound in the heat-sensitive recording materials include phenol compounds, organic acids and salts thereof, and oxybenzoic acid esters, etc. Particularly, phenol compounds are preferably used, because they have a melting point near the desired recording temperature. They have been described in detail in, for example, Japanese Patent Publication Nos. 14039/70 and 29830/76, and U.S. Pat. Nos. 3,244,549 and 3,244,550.
In order to meet requirement (i), one approach has been to elevate the melting point of the electron accepting compound from 60.degree. C. to 100.degree. C. However, in phenolic compounds which have been widely used at present as the electron accepting compounds, it is difficult to control the melting point. Further, the phenolic compounds have little practical value, because they are expensive.
Other approaches have been described in Japanese Patent Publication Nos. 17748/74 and 39567/76. These approaches involve the use of a combination of an organic acid and a phenolic compound as an electron accepting material or the use of polyvalent metal salts of compounds having an alcoholic hydroxy group. Further, it has been described in Japanese Patent Publication No. 29945/76 to use hydroxyethyl cellulose and a copolymer of maleic acid anhydride salt.
Further, the addition of waxes has been described in Japanese Patent Publication No. 27599/76 and Japanese Patent Application (OPI) No. 19231/73 (The term "OPI" as used herein refers to a "published unexamined Japanese Patent Application").
Additional approaches have been described in Japanese Patent Application (OPI) Nos. 34842/74, 115554/74, 149353/75, 106746/77, 5636/78, 11036/78, 48751/78 and 72996/81. These approaches involve adding nitrogen containing organic compounds such as thioacetanilide, phthalonitrile, acetamide, di-.beta.-naphthyl-p-phenylenediamine, aliphatic acid amide, acetoacetanilide, diphenylamine, benzamide oricarbazole, etc., thermoplastic substances such as 2,3-m-tolylbutane or 4,4'-dimethylbiphenyl, etc. or carboxylic acid esters such as dimethyl isophthalate, diphenyl phthalate or dimethyl terephthalate, etc. as a sensitizer.
However, the heat-sensitive recording materials produced by the above described techniques are insufficient with respect to color density and the color sensitivity.